1. Field of the Invention
The invention relates to a superalloy component which has applied thereon a protective coating system with various layers.
2. Description of the Related Art
U.S. Pat. No. 4,055,705 to Stecura et al.; U.S. Pat. No. 4,321,310 to Ulion et al.; and U.S. Pat. No. 4,321,311 to Strangman disclose coating systems for gas turbine components made from nickel or cobalt-based superalloys. A coating system described comprises a thermal barrier layer made from ceramic, which in particular has a columnar grained structure, placed on a bonding layer or bond coating which in its turn is placed on the substrate and bonds the thermal barrier layer to the substrate. The bonding layer is made from an alloy of the MCrAlY type, namely an alloy containing chromium, aluminum and a rare earth metal such as yttrium in a base comprising at least one of iron, cobalt and nickel. Further elements can also be present in an MCrAlY alloy; examples are given below. An important feature of the bonding layer is a thin layer developed on the MCrAlY alloy and used for anchoring the thermal barrier layer. This layer may be alumina or alumina mixed with chromium oxide, depending on the composition of the MCrAlY alloy and the temperature of the oxidizing environment where the layer is developed. Eventually, an alumina layer may be placed purposefully by a separate coating process like PVD.
U.S. Pat. No. 5,238,752 to Duderstadt et al. discloses a coating system for a gas turbine component which also incorporates a ceramic thermal barrier layer and a bonding layer or bond coating bonding the thermal barrier layer to the substrate. The bonding layer is made from an intermetallic aluminide compound, in particular a nickel aluminide or a platinum aluminide. The bonding layer also has a thin alumina layer which serves to anchor the thermal barrier layer.
U.S. Pat. No. 5,262,245 to Ulion et al. describes a result of an effort to simplify coating systems incorporating thermal barrier layers for gas turbine components by avoiding a bonding layer to be placed below the thermal barrier layer. To this end, a composition for a superalloy is disclosed which may be used to form a substrate of a gas turbine component and which develops an alumina layer on its outer surfaces under a suitable treatment. That alumina layer is used to anchor a ceramic thermal barrier layer directly on the substrate, eliminating the need for a special bonding layer to be interposed between the substrate and the thermal barrier layer. In its broadest scope, the superalloy consists essentially of, as specified in weight percent: 3 to 12 Cr, 3 to 10 W, 6 to 12 Ta 4 to 7 Al, 0 to 15 Co, 0 to 3 Mo, 0 to 15 Re, 0 to 0.0020 B, 0 to 0.045 C, 0 to 0.8 Hf, 0 to 2 Nb, 0 to 1 V, 0 to 0.01 Zr, 0 to 0.07 Ti, 0 to 10 of the noble metals, 0 to 0.1 of the rare earth metals including Sc and Y, balance Ni.
U.S. Pat. No. 5,087,477 to Giggins, Jr., et al. shows a method for placing a ceramic thermal barrier layer on a gas turbine component by a physical vapor deposition process comprising evaporating compounds forming the thermal barrier layer with an electron beam and establishing an atmosphere having a controlled content of oxygen at the component to receive the thermal barrier layer.
U.S. Pat. Nos. 5,154,885; 5,268,238; 5,273,712; and 5,401,307, all to Czech et al. disclose advanced coating systems for gas turbine components comprising protective coatings of MCrAlY alloys. The MCrAlY alloys disclosed have carefully balanced compositions to give exceptionally good resistance to corrosion and oxidation as well as an exceptionally good compatibility to the superalloys used for the substrates. The basis of the MCrAlY alloys is formed by nickel and/or cobalt. Additions of further elements, in particular silicon and rhenium, are also discussed. Rhenium in particular is shown to be a very advantageous additive. All MCrAlY alloys shown are also very suitable as bonding layers for anchoring thermal barrier layers, particularly in the context of the invention which will be described in the following.
The above-mentioned U.S. Pat. No. 5,401,307 to Czech et al. also contains a survey over superalloys which are considered useful for forming gas turbine components that are subject to high mechanical and thermal loads during operation. Particularly, four classes of superalloys are given. The respective superalloys consist essentially of, as specified in percent by weight:
1. 0.03 to 0.05 C, 18 to 19 Cr, 12 to 15 Co, 3 to 6 Mo, 1 to 1.5 W, 2 to 2.5 Al, 3 to 5 Ti, optional minor additions of Ta, Nb, B and/or Zr, balance Ni. PA1 2. 0.1 to 0.15 C, 18 to 22 Cr, 18 to 19 Co, 0 to 2 W, 0 to 4 Mo, 0 to 1.5 Ta, 0 to 1 Nb, 1 to 3 Al, 2 to 4 Ti, 0 to 0.75 Hf, optional minor additions of B and/or Zr, balance Ni. PA1 3. 0.07 to 0.1 C, 12 to 16 Cr, 8 to 10 Co, 1.5 to 2 Mo, 2.5 to 4 W, 1.5 to 5 Ta, 0 to 1 Nb, 3 to 4 Al, 3.5 to 5 Ti, 0 to 0.1 Zr, 0 to 1 Hf, an optional minor addition of B, balance Ni. PA1 4. about 0.25 C, 24 to 30 Cr, 10 to 11 Ni, 7 to 8 W, 0 to 4 PA1 5. Ta, 0 to 0.3 Al, 0 to 0.3 Ti, 0 to 0.6 Zr, an optional minor additive of B, balance cobalt. PA1 placing an anchoring layer comprising an oxide compound doped with nitrogen on a substrate formed of a nickel or cobalt-based superalloy; and PA1 placing a ceramic coating on the anchoring layer. PA1 placing a layer comprising an other oxide compound essentially free of nitrogen on the substrate; PA1 establishing an atmosphere containing nitrogen around the layer; and PA1 creating the anchoring layer by subjecting the layer and the atmosphere to an elevated temperature and diffusing the nitrogen into the layer.
Information on modified alumina compounds, in particular alumina compounds doped with nitrogen, is available from an essay by the inventor entitled "Schichtentwicklungen fur Hochtemperaturanwendungen in thermischen Maschinen" [coating developments for high temperature applications in thermic machines] and published under "Fortschritts-Berichte VDI, Reihe 5" [progress reports by VDI, series no. 5], ser. No. 345, VDI-Verlag, Dusseldorf, Germany, 1994. That essay also contains information about processes to deposit such alumina compounds in the form of layers.
Further information on modified alumina compounds may be derived from an essay by L. Peichl and D. Bettridge entitled "Overlay and Diffusion Coatings for Aero Gas Turbines" and contained in a book entitled "Materials for Advanced Power Engineering, Part One", edited by D. Coutsouradis et al., Kluwer Academic Publishers, Dordrecht, Netherlands, 1994, pages 717-740, and from an essay by O. Knotek, E. Lugscheider, F. Loffler and W. Beele, published in: Surface and Coating Techniques, Vol. 68/69 (1994), pages 22 to 26.
A standard practice in placing a thermal barrier coating on a substrate of an article of manufacture includes developing an oxide layer on the article, either by placing a suitable bonding layer on the article which develops the oxide layer on its surface under oxidizing conditions or by selecting a material for the article which is itself capable of developing an oxide layer on its surface. That oxide layer is then used to anchor the thermal barrier layer placed on it subsequently.
Under thermal load, diffusion processes will occur within the article. In particular, diffusion active chemical elements like hafnium, titanium, tungsten and silicon which form constituents of most superalloys used for the articles considered may migrate through the oxide layer and into the thermal barrier layer. The diffusion active chemical elements cause damage to the thermal barrier layer by modifying and eventually worsening its essential properties. That applies in particular to a thermal barrier layer made from a zirconia compound like partly stabilized zirconia, since almost all zirconia compounds must rely on certain ingredients to define and stabilize their particular properties. The action of such ingredients is likely to be imparted by chemical elements migrating into a compound, be it by diffusion or otherwise.
To assure that a thermal barrier layer placed on a substrate containing diffusion active chemical elements keeps its essential properties over a time period as long as may be desired, it is therefore material to prevent migration of diffusion active chemical elements into the thermal barrier layer.
That aspect has, however, not yet received attention in this art. Heretofore, only oxide layers have been given consideration to anchor a thermal barrier layer on a superalloy substrate regardless of their transmission of diffusing chemical elements to the thermal barrier layer.